This invention relates generally tog round fault detectors for direct current power supplies.
It is known to include in high-voltage DC power supplies a ground fault detector which measures leakage current to ground. Once the leakage current reaches a predetermined value, an alarm is provided or a warning system or the high-voltage circuits of the power supply are automatically disconnected to prevent circuit errors or a possible hazardous condition.
In one type of prior art ground fault detector, the DC leakage current is measured, and when it reaches a level indicating an unacceptable ground fault condition, the mechanism is activated to provide a warning or to disconnect the high power circuits. It has the disadvantages of not detecting faults unless the high-voltage is already on.
Another type of ground fault detector measures AC fault impedance without the high-voltage being on. This type of ground fault detector is disclosed in U.S. Pat. No. 4,188,574.
This type of prior art ground fault detector has the disadvantage of being set at a level corresponding to a AC leakage current. Thus, it is activated unnecessarily by AC leakage current when the capacitance becomes large, and from time to time unnecessarily interrupts the operation of the power supply and equipment to which it is supplying power. The activation level is set too low because it is not set in accordance with the circuit conditions providing the largest safe capacitive charge.
An alternative to having the ground fault circuit activated unnecessarily in this type of prior art ground fault detector is to set the current level higher but this has the disadvantage of permitting an unacceptable DC leakage current when the leakage capacitive impedance is high.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a novel ground fault detector.
It is a further object of this invention to provide a novel method for indicating ground faults.
It is still a further object of the invention to provide a novel ground fault detector for a high-voltage current that senses fault impedance, rather than current, so that ground faults are detected before the high-voltage is turned on.
It is a still further object of the invention to provide novel apparatuses and methods for detecting ground faults, which methods and apparatuses can be easily adjusted to indicate predetermined dangerous conditions across a wide range of capacitance to ground.
It is a still further object of this invention to provide a novel method and apparatus for detecting ground faults for DC power supplies by setting the condition for activation of the detector at a value of the real component of an AC impedance.
It is a still further object of the invention to provide a novel apparatus and method for detecting ground faults which measure the real component of AC leakage current and provide a warning or to terminate the high power when the real component of AC leakage current exceeds a predetermined amount.
It is a still further object of this invention to provide a novel apparatus and method for detecting the magnitude of the AC leakage current which is in phase with the AC high-voltage of the power supply.
It is a still further object of this invention to enable a ground fault level of impedance to be set as a standard in accordance with proper operating circuit conditions that permit the largest leakage current.
It is a still further object of the invention to provide a ground fault detection technique which automatically adjusts a permissible level of AC test leakage to take into consideration the acceptable circuit configurations that provide large reactance or large resistance.
In accordance with the above and further objects of the invention, a ground fault detector for a DC power supply measures the real component of the AC leakage current. The real component is compared against a preset level. When it exceeds that level, thus indicating an impermissible low impedance to AC ground from the high-voltage end of the power supply, the ground fault detector is activated. Upon activation, the ground fault detector gives an alarm or a series of alarms or disconnects the high-voltage.
Advantageously, the real component of AC impedance to ground is detected by generating a voltage which represents the AC leakage current and selecting a portion of that voltage which corresponds in phase to the AC potential of the power supply. The signal representing the real component of AC leakage current is compared against an adjustable, preset, level and, when the level is exceeded, any of several selected events may be caused to happen, such as: (1) an audible or visual indication of a ground fault may be provided; and/or (2) the power supply or the high voltage circuit of the power supply may be deenergized.
For further beneficial results, this preset level or the signal indicating the real component of AC leakage impedance can be corrected for shifts in the voltage output of the high-voltage end of the transformer and may be corrected to accommodate different ranges of voltage output of the power supply if such ranges are selected.
As can be understood from the above description, the method and apparatus for detecting ground faults in accordance with this invention has several advantages, such as: (1) it permits the leakage capacitance to AC ground to vary over a wide range without causing the ground fault condition to be activated; (2) it permits the power supply to be used in higher leakage capacitance configurations or with high capacitance equipment; (3) it is easily adjustable and adaptable to different circuit conditions; and (4) it enables the level of impedance to AC ground that is acceptable to have large capacitive AC leakage current and it does this by increasing the permissible value of admittance that is measured as the phase difference is increasedby leakage capacitance.